Internal combustion engines may include water injection systems that inject water from a storage tank into a plurality of locations, including an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Injecting water into the engine intake air may increase fuel economy and engine performance, as well as decrease engine emissions. When water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to the water. This heat transfer leads to evaporation, which results in cooling. Injecting water into the intake air (e.g., in the intake manifold, ports, etc.) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx, while a more efficient fuel mixture may reduce carbon monoxide and hydrocarbon emissions.
As explained above, water may be injected into different locations, including the intake manifold, intake ports of engine cylinders, or directly into engine cylinders. However, the inventors have recognized that water injection benefits may be limited based on the location of the water injection as well as the engine operating conditions at the time of the water injection. As an example, manifold water injection may be used to provide charge cooling. However the charge cooling benefit may be limited when the ambient humidity is elevated. As another example, manifold water injection may be used to provide charge dilution. However, the injected water may puddle if the water does not evaporate rapidly enough, leading to potential misfires. If the water injection benefits are not sufficiently leveraged, fuel economy and engine stability may be degraded.
In one example, the issues described above may be addressed by a method for an engine comprising, during a first condition, responsive to an engine dilution demand, port injecting water towards a closed intake valve; and during a second condition, responsive to engine knock, port injecting water away from an open intake valve. In this way, port water injection may be utilized both for increasing engine dilution to decrease pumping losses and increase charge air cooling to reduce engine knock and increase engine efficiency by adjusting the direction and timing of the injection.
As one example, an engine may be configured with a first set of port injectors angled towards the intake valve, and a second set of port injectors angled away from the intake valve (e.g., towards the intake manifold). During conditions when engine combustion is knock limited, such as at high loads, water may be injected via the second set of port injectors at a timing when intake valves are open. This results in a larger portion of the water being injected in the liquid form, improving the charge cooling effect of the injection. In comparison, during conditions when engine combustion is dilution limited, such as at low loads, water may be injected via the first set of port injectors at a timing when intake valves are about to be closed, and while the valve surface is hot. This results in a flash vaporization of the water when it impinges the surface. Consequently, a larger portion of the water injected will rapidly change into vapor form, improving the charge diluting effect of the injection. In addition, the water injection via the first set of injectors may be sensed via dilution (or concentration) sensors (e.g. IAO2) while the water injection via the second set of injectors may be sensed via temperature sensors.
In this way, distinct water injection benefits may be provided in an engine at a given water injection location by varying a direction and timing of the injection. The technical effect of port injecting water on the hot surface of a closed intake valve, such as at BDC of an intake stroke, in the same direction as air flow, is that the injected water can be substantially immediately evaporated. As a result, a charge dilution effect of the water injection can be increased while the charge cooling effect of the water injection is decreased. By port injecting the water when the valve surface is hot, water puddling is reduced, reducing the risk of water induced misfires. The technical effect of port injecting water away from an open intake valve, in the opposite direction of air flow, is that the turbulence from the high speed of airflow can be advantageously used to improve atomization of the injected water into the air before the air-water mixture is delivered to the engine cylinder. As a result, a charge cooling effect of the water injection can be increased while the charge dilution effect of the water injection is decreased. By selecting a water injection sensing mode based on the water injection benefit being leveraged (e.g., charge cooling or dilution), a water injection error may be more reliably determined and compensated for. Overall, water injection benefits may be extended over a wider range of engine operation, improving engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.